A hip joint acetabular prosthesis cup of this type is known, for example, from the German AS (Publication for Opposition) No. 24 11 617. In the case of this known cup, besides the internal diameter or root diameter of the thread, also constantly decreasing is the outer diameter of the thread--at the outer edges of the thread profile--toward the face or crown surface of the cup; the thread profile corresponds to the thread profile of the bone screw and remains approximately constant over the entire axial length of the thread in order to enable easy emplacement of the cup into the correspondingly routed-out acetabulum, without first cutting a thread in the bony tissue. Additionally, this known cup has cutouts, running from the cup rim toward the crown, into which, after implantation of the cup, the bony tissue should grow in order to improve anchoring the cup. However, it has been demonstrated that the firmness of these cementlessly installed type cups is not always quite satisfactory, particularly after longer period of use. In particular, anchoring in the region of the crown of the cup, where the natural or prosthesis femoral head is supported, often times can not withstand the considerable forces introduced into the pelvic bones over the cup, whereby undesired loosening phenomena can occur over a shorter or longer period of time.
Known from the German OS (Laid-Open Print) No. 29 11 754 or OS No. 26 45 101, are hip joint acetabular prothesis cups that display cutouts running from the cup rim toward the crown, into which, after implanting, the bony tissue is said to grow in order to increase adherence of the cup in the bony tissue.
As compared to this, the object of the invention is to further develop a hip joint acetabular prosthesis cup of the initially mentioned art such that the bearing surface between the cup and surrounding bony tissue is enlarged.
This object is resolved in accordance with the invention by the fact that the outer diameter of the thread is approximately constant over the entire axial length of the thread.
The advantages of the invention rest particularly in the fact that the outer edge of the thread lies essentially over a circular cylinder, while the inner diameter or root diameter continually decreases toward the crown of the cup, so that the carrying (bearing) surface of the thread profile continually increases from the circular cup rim up to the crown-side end of the thread. The bony tissue that is available about the acetabulum cavity in the pelvic bone is, in this manner, utilized quite well for forming the form-closed cup/bony tissue bond, and achieves and effective interengagement of bony substance and thread. Because of the increasing thread profile toward the crown, it is possible, if required, also to decrease the axial length of the thread, and it is also possible to achieve good anchoring in a comparatively flat pelvis, whereas the known type hip cup would project from the bony tissue with part of its thread.
Preferably, the thread has a non-symmetrical profile with a thread outer edge that is offset, in the axial direction of the thread, toward the crown of the cup. Particularly preferred here is that the thread profile be convexly curved away from the plane of the annular cup edge.
Hence, in this form of embodiment, the outer edge profile is at a greater distance from the plane of the cup edge than is the heel (beginning point) of the profile. This form of embodiment has the advantage that the cup remains firmly anchored in bony tissue because of the weight-occassioned loading and the large force components in the thread axis direction that are available thereby, and does not press the surrounding bony tissue radially outwardly and thereby become loose, like in the case of known symmetrical profiles.
According to a particularly preferred form of embodiment of the invention, the outer cup has externally lying cutouts that break through the thread, increase the elasticity of the cup and, for another thing, grow full with bony tissue, thereby preventing the cup from rotating itself out. In accordance with the invention, the cutouts are disposed at an acute angle on the external surface of the cup relative the plane of the cup edge; they therefore run, for example, over large circles that run, at some lateral distance away, past the crown of the cup.
In this fashion, anchoring of the thread is better distributed over the entire periphery of the cup than in the case of known cups that have cutouts running through the crown over large circles.
Advantages of further evolutions of the invention are characterized by the features of the subclaims.